[mrdreex]

Application:

- 383 SBC
- AFR 210 Comp. Ported cylidner heads
- Solid Roller, 258/263 @ .050", .668" with 1.6's
- 4-speed Borg Warner Super T10
- 3400lb '72 Nova (guessing here)
- 8.5 GM Big 10 bolt, 4.11 gears
- 750 Mighty Demon, Annular Boosters (I probably should go bigger, but this can work for now)

I am having some tuning issues. Symptoms include: what seems like a VERY bad bog and general just sluggishness in every gear. It would seem as if it's too rich, but my idle mixture screws are 3/4 a turn in! I'm confused, as it is my thinking that an off idle bog indicates an over-rich condition..? The manual states that it should be between 1 - 2.5 turns out. Can someone clarify how I should go about tuning this?

I followed the advice given here:
http://moparchat.com/forums/showthread.php?t=56317

..but am unsure if all of it apply to SBC's, or my application (would imagine so, can't think of why it wouldn't). I followed the directions given here, but got lost/stuck at the part where it mentions going in 1/8 turn after hitting the accelerator pump. When I do, my engine just DIES (yet again, indicating an over-rich condition). I don't want to lean it out too much and am coming here for some reassurance. The only way I could get my car to idle correctly was to set the butterflys open way more than .020 - .030", and am curious as to if that would cause the stumble..but I can't get the car to idle right at all without the butterflys being open. The carb is not equipped with Idle-Eze it would seem, as I cannot get the screw in the middle to turn (it may not be a screw, it's really hard to tell)

I do want to tell you that I am new to tuning carburetors. There is no snap in the throttle when I stomp on it. Every gear that I go in, there is a stumble (idle into 1st, 2nd, 3rd, and 4th). Any help from you guys is appreciated, and maybe [email protected] can help me out. I know he has been a help before.

..any help is appreciated. Point me in the right direction please..

 

[67-4-fun]

hope i can give you some insight hopfully, Are your plug's black??? this be a good indicator as if your running rich, also are you getting black smoke from the exaust when you rev it up? also take your idle mixture screws and screw them in till flush do not over tighen and back off 1.5 out thats a good starting point on the primary side and secondary side for starters.

also need alittle more info, Timing set at? jets used, accelerater pumps and pum cam used{ just color of cam} and shooters used that will tell me alittle but more to maybe help get you in the right direction. if you have trouble locating these items send me a PM and I'll explane. I'm not an expert but hope i can help

 

[cobalt327]

What is the vacuum at idle, and what power valve is in it? What is the fuel pressure?

What about the timing- you need a lot of initial w/that cam, IMO.

Are you trying to use vacuum advance? If so, ported or manifold vacuum?

What type of intake manifold?

 

[67-4-fun]

your carb is a 4 corner idle system is it??? no vacum secondaries??? if my memory seves me right.

 

[mrdreex]

hope i can give you some insight hopfully, Are your plug's black??? this be a good indicator as if your running rich, also are you getting black smoke from the exaust when you rev it up? also take your idle mixture screws and screw them in till flush do not over tighen and back off 1.5 out thats a good starting point on the primary side and secondary side for starters.

also need alittle more info, Timing set at? jets used, accelerater pumps and pum cam used{ just color of cam} and shooters used that will tell me alittle but more to maybe help get you in the right direction. if you have trouble locating these items send me a PM and I'll explane. I'm not an expert but hope i can help

Hey 67,

Plugs are not black. I will provide some pictures tomorrow. They are tan, but at idle, it is really rich, which leads me to believe I have a power valve issue. I do not know for certain what its rating is, but I haven't changed a thing since I bought it. I think it's 6.5 stock. There is no black smoke when I rev it, but letting it idle in the garage, it gets to the point where it really gets to the eyes, and hurts to breathe it in (that's with the 4-corner idle screws out 1.5 turns, and yes, it's a 4-corner idle system), indicating realllllly rich. This thing has me going all over the place.

I was in a hurry earlier, forgot to post what my timing is. At idle it's ~20 degrees, and ~36, all in by 2250 or so. I really should be running it locked out. No vacuum advance. Static C/R is 10.8:1 (we'll call it 11:1). Fuel pressure is 6.5, and the fuel bowls are set fine. It comes up to the halfway on the sight glasses, and none is coming out of the vent-tubes.

The carb does have mechanical secondaries. It has a red pump cam, both primaries and secondaries. You will have to excuse me on the jet sizes, as I have no idea, and I have never changed one before (nor a power valve). I do my homework though, so any advice is appreciated. I will probably be looking at it alot closer tomorrow, and will get some pictures up.

Thanks guys.

 

[67-4-fun]

By your post it looks like you fuel level in you float bowls maybe a little high, I recomend them being at or just a tick below the sight plus hole, i run mine right at the bottom of the of the hole to where if you take the plug out a little trinkle of gas may run out, but not alot. try this ajustment first and then see if your rich idle lean's a bit, but if no changes acured, then you need to check your manifold vacum at idle with car in gear{ with an auto trans} and get a reading to tell if you need another PV. start hear and well go from there. dont want to make to many ajustments at one time so we can do an elimination process.

JON

 

[mrdreex]

By your post it looks like you fuel level in you float bowls maybe a little high, I recomend them being at or just a tick below the sight plus hole, i run mine right at the bottom of the of the hole to where if you take the plug out a little trinkle of gas may run out, but not alot. try this ajustment first and then see if your rich idle lean's a bit, but if no changes acured, then you need to check your manifold vacum at idle with car in gear{ with an auto trans} and get a reading to tell if you need another PV. start hear and well go from there. dont want to make to many ajustments at one time so we can do an elimination process.

JON

Jon,

Thanks for the help. Last time I checked vacuum at idle, it was around 8-9 inches, but it's been a while since I checked. I won't be able to check until later this week unfortunately because, upon further inspection of valvetrain noise, I discovered I had broken a rocker arm. Luckily the only damage was to the rocker arm roller-tip and I was able to retrieve both the pin and the roller. I think this contributed to it running somewhat bad, but I doubt the bog was due to that.

In the meantime, I will adjust the float bowls to the bottom of the sight glass, and I will look to see what jets/PV I am running.

Thanks.

 

[cobalt327]

upon further inspection of valvetrain noise, I discovered I had broken a rocker arm.

Who made the rockers?

 

[mrdreex]

Who made the rockers?

Crower, aluminum full-roller (#72841 on their site). I looked at all the others, and the pins looked just fine. The only other thing is slight discoloration of the retainer, but no deformities nor scuffs.

 

[mrdreex]

I pulled off the float bowl tonight and looked at my power valve/jets. I can't tell what size PV it is, because it has both the number 6 AND 7 on either side...really confusing

As far as the jets are concerned, both are #70's. If I go lower, what should I start out with, and what should I look out for?

Thanks.

 

[67-4-fun]

not sure on jetting as maybe leave it for now,reset the float bowls as described and for the PV goes try a 6.5 first then maybe go to a 5.5 and see if you have any changes, keep the idle mixture screws were there at for now, if it does not want to idle, turn idle up slightly both ends keeping them equal { not the mixture screws}, if idles fine then leave it, then try it out and see what it's going to do, As for the post you sent the link to on how to tune the demon 750 carb, I read it and understand what he is doing to a point, but its hard to explane it, to ya with out talking to ya one on one, it's hard with a key board...

 

[mrdreex]

not sure on jetting as maybe leave it for now,reset the float bowls as described and for the PV goes try a 6.5 first then maybe go to a 5.5 and see if you have any changes, keep the idle mixture screws were there at for now, if it does not want to idle, turn idle up slightly both ends keeping them equal { not the mixture screws}, if idles fine then leave it, then try it out and see what it's going to do, As for the post you sent the link to on how to tune the demon 750 carb, I read it and understand what he is doing to a point, but its hard to explane it, to ya with out talking to ya one on one, it's hard with a key board...

Thanks 67,

I ordered a new 6.5 power valve, as I'm not completely sure what's in it right now. This will provide a good baseline at the very least. I will buy jets individually as they are needed.

I followed the guide, interpreting it as I will try and explain.

1) Stuck paper in between both primary/secondary idle speed screws until I could barely pull the paper out, then turned in 1/8 of a turn.

2) I bottomed out all 4 mixture screws, and then backed out 1.5 turns (3 half turns).

3) I then proceeded to try and turn the car on. It was very hard to get started as the idle speed is way under what it will idle at. It wanted to idle around 400-500 RPM, but, this car will not idle (and will quickly stall) at anything under 800, if not tapping the throttle.

4) I may have interpreted this part wrong. It says that, once you get it running, to hold the throttle open, and do NOT let the car die, and to ease down the throttle/butterflys to the stops to the point that it does not stall out. I then pressed down the primary squirter, and bam, the car died instantly. That tells me it was too rich. Now, first time I read it, I thought I needed to go in on the mixture screws. But, after re-reading it, it said to adjust by 1/8-1/4 turn, the idle speed screws.

This is kind of confusing, because earlier in the guide, it mentions that the idle transfer slot should be only ~.020" shown. When adjusting the idle speed, it opens the butterflys, thus showing more of the transfer slot.

Can anyone explain that a little bit more? Thanks for any help.

 

[cobalt327]

This is kind of confusing, because earlier in the guide, it mentions that the idle transfer slot should be only ~.020" shown. When adjusting the idle speed, it opens the butterflys, thus showing more of the transfer slot.

Can anyone explain that a little bit more?

I don't know what you know, so bear w/me.

You may find that you need to open the primary blades too far w/the idle speed screw, in an attempt to get enough idle speed. The transfer slot can't be too exposed, or the idle mixture screws become ineffective and the off-idle transition will be poor.

In this case you can try tipping the 2'ndary blades open slightly more by using their adjustment screw- to let in some more air. Then readjust the idle mixture screws, and curb idle speed as needed.

There are some cases where nothing seems to provide a decent idle speed/quality. In these few cases, you may need to drill the primary blades, but you are not there, yet- so resist taking that route until you're sure that's all that's left.

 

[mrdreex]

In this case you can try tipping the 2'ndary blades open slightly more by using their adjustment screw- to let in some more air. Then readjust the idle mixture screws, and curb idle speed as needed.

That's how I had it running at one point. I read somewhere that that's how you would compensate for a high idle. I adjusted the secondaries to compensate for the lack of air speed, but saw conflicting ideas in that guide, where it mentioned to do the same to both idle speed screws.

It ran ok, but it still bogged.

You mention to readjust the mixture screws. Would adding more air into the secondaries (turning in the idle speed screws) imply only adjusting the secondary mixture screws (back 2), and richening the mixture?

Thanks for the help.

 

[cobalt327]

You mention to readjust the mixture screws. Would adding more air into the secondaries (turning in the idle speed screws) imply only adjusting the secondary mixture screws (back 2), and richening the mixture?

In this case, I'm saying to "cheat" and add mixture volume w/the front idle screws- if adding just air through opening the 2'ndary isn't enough.

If you find yourself having to go real far from "normal"- from about 1 to say, 2-1/2 or 3 turns out, that there may need to be other adjustments made like secondary opening or air bleeds (they are adjustable, right?) in order to bring the mixture screws back around.

Another tact, is to set the primary blades correctly- 0.020"- 0.030" or whatever the specs are for the transfer slot exposure. From there, add 2'ndary opening, front mixture screw settings and work w/those (and the 2'ndary idle mixture screws, as needed) in order to establish a decent idle.

 

[mrdreex]

If you find yourself having to go real far from "normal"- from about 1 to say, 2-1/2 or 3 turns out, that there may need to be other adjustments made like secondary opening or air bleeds (they are adjustable, right?) in order to bring the mixture screws back around.

How do I figure out what 'normal' is? The initial 1.5 turns out? Do I adjust the mixture screws out until the car wants to die (due to being too rich) and go in from there? If so, how much is enough? I know a wideband would help tuning by A/F ratio, but that's not in the budget at the moment. Yes, the air bleeds, I believe, are removable/adjustable on a Mighty Demon.

Excuse my ignorance on this, it's my first time having to really get down and fine tune a carb. There are lots of bells and whistles to this.

Thanks for your help/patience.

 

[cobalt327]

How do I figure out what 'normal' is? The initial 1.5 turns out?

Yes, in this area. I find that usually the initial setting to be a little rich.

Do I adjust the mixture screws out until the car wants to die (due to being too rich) and go in from there? If so, how much is enough?

The idea would be to have the mixture screws out too far for how the primary blades have to be set (because if you open them more, too much transfer slot would be exposed, and you can't go that far). So at this point, the engine will be rich.

Then give the engine more air (lean it out) by tipping the secondary blades open some more- with the adjustment they have.

Leave the 2'ndary mixture screws where they are, for now- unless they're real far in or out. If they are radically different from what the baseline is, let us know.

I know a wideband would help tuning by A/F ratio, but that's not in the budget at the moment. Yes, the air bleeds, I believe, are removable/adjustable on a Mighty Demon. An O2 sensor would be great- but is not necessary to get this deal tuned in pretty close.

See what the above gets you, then we'll see what is up w/the bleeds.

 

[mrdreex]

Then give the engine more air (lean it out) by tipping the secondary blades open some more- with the adjustment they have.

Leave the 2'ndary mixture screws where they are, for now- unless they're real far in or out. If they are radically different from what the baseline is, let us know.

So, basically I would be using the primary mixture screws to compensate for the 'leaning out' (adding more air speed via idle screw) to the secondaries, instead of adjusting the secondary mixture screws themselves? Is the reasoning behind this so that when the secondaries crack open ('flooring' it), it's not over-rich and cause a bog, with a dual purpose of compensating for the lean-out?

How would the secondary mixture screws be radically different than baseline setting if I am not to touch them?

Thanks again cobalt327, I'm slowly understanding this.

 

[McLeay F1]

Rule of thumb with this type of carburetor, in my experience, if your vacuum gage is reading 8 to 9 inches of vacuum at idle, you need a 4.5 power valve. Once you have done that. Setting mixture screws is easy, with or with out a vacuum gage,

My way of doing it is this; have someone sit in your car, put it in gear with the brakes on. Then one screw at a time turn the screw in slowly, until the engine wants to die, back out slowly until it smoothes out or attains highest vacuum. Once that’s done no need to touch again if it has a four corner air idle screw adjustment, same applies at all four corners. Careful not to bottom out air idle screws “harshly” can cause damage to screw and the inside bottom of the hole.

If you have the carb off the intake, hold carburetor over your head, look too see how much light is showing on secondary plates it should be just a crack of light showing, if you have a fairly radical camshaft open up slightly with secondary throttle blade adjustment screw. . Hope that helps
:sweat:

 

[cobalt327]

So, basically I would be using the primary mixture screws to compensate for the 'leaning out' (adding more air speed via idle screw) to the secondaries, instead of adjusting the secondary mixture screws themselves?

Yes. It's more about the "volume" than "speed" in this instance. But more is the operative word.

Is the reasoning behind this so that when the secondaries crack open ('flooring' it), it's not over-rich and cause a bog, with a dual purpose of compensating for the lean-out?

Yes. Really, this is an effort to reach a decent idle quality along w/an increase in vacuum, without reaching the "too far" point, where the transfer slot has become too exposed because the primary idle speed screw has opened the primary blades too far. That HAS to be avoided- and this is all about getting enough airflow w/o opening the primaries too much.

Please understand that the way we're going about this is unorthodox, to put it mildly. But it should be as effective as going back to line one of the tuning manual- a place where we might end up anyway!

How would the secondary mixture screws be radically different than baseline setting if I am not to touch them?

Because of where the previous tuner may have set them. You should carefully count the turns it takes to lightly seat them and then return the setting to where it was. Write this down (and write down all other subsequent changes as they're done- pri. and sec. both), because it will be a bit much to remember it all in your head. That way, there will be no question of what was done.

Thanks again cobalt327, I'm slowly understanding this.

Like I've said- this isn't necessarily the "approved" method, here. But the results are what matters, not how you got there.

The changes that you're now making can be put back to where they were previously- easily- should there be a need. And what we're doing here has as much to do w/determining what's the issue, as resolving the issue.